Process for the preparation of urea

ABSTRACT

Process for the preparation of urea from ammonia and carbon dioxide, wherein the waste gases released at virtually atmospheric pressure in a urea plant are compressed by an ejector driven by the waste gas from the high-pressure synthesis and are supplied to a medium-pressure absorber. The pressure of the waste gases is increased by 0.15-1 MPa.

[0001] The invention relates to a process for the preparation of ureafrom ammonia and carbon dioxide.

[0002] Urea may be prepared by introducing excess ammonia along withcarbon dioxide into a synthesis zone at a suitable pressure (for example12-40 MPa) and a suitable temperature (for example 160-250° C.), whichfirst results in the formation of ammonium carbamate according to thereaction:

2NH₃+CO₂→H₂N—CO—ONH₄

[0003] Dehydration of the ammonium carbamate formed then results in theformation of urea according to the equilibrium reaction:

H₂N—CO—ONH₄←→H₂N—CO—NH₂+H₂O

[0004] The theoretically attainable conversion of ammonia and carbondioxide into urea is determined by the thermodynamic position of theequilibrium and depends on for example the NH₃/CO₂ ratio (N/C ratio),the H₂O/CO₂ ratio and temperature.

[0005] In the conversion of ammonia and carbon dioxide to urea in thesynthesis zone, a reaction product is obtained from the synthesisreactor which product is a urea synthesis solution which consistsessentially of urea, water, ammonium carbamate and unbound ammonia.

[0006] Besides the aforementioned urea synthesis solution, there mayevolve in the synthesis zone a gas mixture of unconverted ammonia andcarbon dioxide along with inert gases, which gas mixture is also knownas synthesis gas. The inert gases present in it may originate from forexample a system that adds air to the plant in order to improve theplant's corrosion resistance. For example, inert gaseous components maybe supplied to the synthesis zone via the raw materials (NH3 and CO2).Ammonia and carbon dioxide are removed from the synthesis gas and arepreferably returned to the synthesis zone.

[0007] The synthesis zone may comprise separate zones for the formationof ammonium carbamate and urea. These zones may, however, also be unitedin a single apparatus. The synthesis may be effected in a single reactoror in two reactors. If two reactors are employed, the first reactor, forexample, may be operated with virtually fresh raw materials and thesecond with raw materials that are completely or partly recirculatedfrom for example the urea recovery.

[0008] The conversion of ammonium carbamate into urea and water in thesynthesis reactor can be effected by ensuring a sufficiently longresidence time for the reaction mixture in the reactor. The residencetime will in general be longer than 10 min, preferably longer than 20min. The residence time will in general be shorter than 3 hours,preferably shorter than 1 hour.

[0009] In practice, various processes are used for the preparation ofurea. Initially, urea was prepared in so-called conventionalhigh-pressure urea plants, which at the end of the 1960s were succeededby processes carried out in so-called urea stripping plants.

[0010] A conventional high-pressure urea plant is understood to be aurea plant in which the decomposition of the ammonium carbamate that isnot converted into urea and the expulsion of the customary excessammonia take place at a substantially lower pressure than the pressurein the synthesis reactor itself. In a conventional high-pressure ureaplant the synthesis reactor is usually operated at a temperature of180-250° C. and a pressure of 15-40 MPa. In a conventional high-pressureurea plant, following expansion, dissociation and condensation at apressure of between 1.5 and 10 MPa, the raw materials that are notconverted into urea are returned to the urea synthesis as an ammoniumcarbamate stream. In addition, in a conventional high-pressure ureaplant, ammonia and carbon dioxide are fed directly to the synthesisreactor. The N/C ratio in the urea synthesis in a conventionalhigh-pressure urea process is between 3 and 5.

[0011] Initially, such conventional urea plants were designed asso-called ‘Once-Through’ processes. Here, non-converted ammonia wasneutralised with acid (for example nitric acid) and converted intoammonium salts (for example ammonium nitrate). It did not take longuntil these conventional Once-Through urea processes were replaced withConventional Recycle Processes, in which non-converted ammonia andcarbon dioxide are recycled to the synthesis reactor as ammoniumcarbamate streams. In the recovery section, non-converted ammonia andcarbon dioxide are removed from the urea synthesis solution obtained inthe synthesis reactor, in which process a urea in water solutionevolves. Next, this urea in water solution is converted into urea in theevaporation section by evaporating water at reduced pressure. Sometimes,the urea-water mixture is separated by means of crystallizationtechniques.

[0012] A urea stripping plant is understood to be a urea plant in whichthe decomposition of the ammonium carbamate that is not converted intourea and the expulsion of the customary excess ammonia largely takeplace at a pressure that is essentially virtually equal to the pressurein the synthesis reactor. This decomposition/expulsion takes place in astripping zone with or without addition of a stripping gas. In astripping process, carbon dioxide and/or ammonia may be used asstripping gas before these components are added to the synthesisreactor. Such stripping is effected in a stripper installed downstreamof the synthesis reactor; in it, the urea synthesis solution coming fromthe synthesis reactor is stripped with the stripping gas with additionof heat. It is also possible to use thermal stripping here. Thermalstripping means that ammonium carbamate is decomposed and the ammoniaand carbon dioxide present are removed from the urea solutionexclusively by means of the supply of heat. Stripping may also beeffected in two or more steps. In a known process a first, purelythermal stripping step is followed by a CO2 stripping step with furtheraddition of heat. The ammonia and carbon dioxide-containing gas streamexiting from the stripper is returned to the reactor whether or not viaa high-pressure carbamate condenser. Stripping of the urea synthesissolution with a stripping agent may take place in more than onestripper.

[0013] In a urea stripping plant the synthesis reactor is operated at atemperature of 160-240° C., preferably at a temperature of 170-220° C.The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-19.5MPa. The N/C ratio in the synthesis zone in a urea stripping plant isbetween 2.5 and 4.

[0014] A frequently used embodiment for the preparation of urea by astripping process is the Stamicarbon CO₂ stripping process as describedin European Chemical News, Urea Supplement, of Jan. 17, 1969, pages17-20. The greater part of the gas mixture obtained in the strippingoperation is condensed and adsorbed, together with the ammonia requiredby the process, in a high-pressure carbamate condenser, after which theammonium carbamate stream that has formed here is returned to thesynthesis zone for the formation of urea.

[0015] The high-pressure carbamate condenser may de designed as, forexample, a so-called submerged condenser as described in NL-A-8400839.The submerged condenser can be placed in horizontal or verticalposition. It is, however, particularly advantageous to carry out thecondensation in a horizontal submerged condenser (a so-called poolcondenser; see for example Nitrogen No 222, July-August 1996, pp.29-31), because, in comparison with other designs of this condenser, theliquid usually has a longer residence time in the pool condenser. Thisresults in the formation of extra urea, which raises the boiling point,so that the difference in temperature between the urea-containingammonium carbamate solution and the cooling medium increases, resultingin better heat transfer.

[0016] After the stripping operation, the pressure of the stripped ureasynthesis solution is reduced to a low level in the urea recovery andthe solution is concentrated by evaporation, after which urea isreleased and a low-pressure ammonium carbamate stream is recirculated tothe synthesis section. Depending on the process, this ammonium carbamatemay be recovered in either a single or a plurality of process stepsoperating at different pressures.

[0017] In the urea process, waste gases are formed as by-products invarious locations in the plant. These waste gases are essentially inertgases, contaminated with ammonia and carbon dioxide, whose inertconstituent components are vented to the atmosphere. Prior to beingremoved from the plant, these waste gases should first be rid of ammoniain particular.

[0018] The waste gases are often cleaned by an absorption step in whatis known as an absorber, in which especially ammonia is removed from thewaste gases with the aid of a suitable ammonia solvent such as water orweakly ammoniacal process condensate, with or without heat being removedby for example heat exchangers. Waste gases are formed at variouspressure levels. It is advantageous for ammonia to be absorbed fromthese waste gases at as high a pressure as possible.

[0019] In the Stamicarbon CO₂ stripping process, for example, the gasstream from the reactor, the synthesis gas, is scrubbed in a scrubber athigh pressure (>10 MPa) using an ammonium carbamate solution originatingfrom the low-pressure section of the plant. In the high-pressurescrubber an ammonium carbamate stream is formed, which stream normallyis passed to the high-pressure condenser and a non-condensed stream ofwaste gases. Prior to being vented to the atmosphere, the waste gasesfrom the high-pressure scrubber are purified of the remaining ammonia byan absorption step in an absorber operating at medium pressure. ‘Atmedium pressure’ means at a pressure of for example 0.3-0.6 MPa.

[0020] Other examples in which waste gases are purified by an absorptionstep include the ‘Self-stripping process’ as described in Uhlmann'sEncyclopedia of Industrial Chemistry, Vol A 27, pages 346-348, 1996, inwhich the waste gases from the high-pressure section of the plant areeventually also scrubbed in an absorber operating at medium pressure.Likewise, in the ACES process, as described Uhlmann's Encyclopedia ofIndustrial Chemistry, Vol A 27, pages 348-349, 1996, the waste gases,containing non-condensable components, vented from the high-pressurescrubber to the medium-pressure decomposition stage are eventuallyscrubbed in an absorber operating at medium pressure.

[0021] According to the current state of the art, the waste gases fromthe high-pressure section of the plant are passed to the medium-pressureabsorber via control valves. A serious drawback is that the energy thatpotentially may be recovered is lost in the process.

[0022] Furthermore, ammonia-bearing gas streams are released in all ureaplants at much lower pressures of between for example 0.1 MPa and 0.3MPa, but mostly at atmospheric pressure. An example is the waste gas inthe condensation zone of the evaporation section. In the evaporationsection, the urea solution is concentrated by evaporation at reducedpressure, in which process there is formed a water vapour streamcontaminated with ammonia from which the condensable components arecondensed in a condenser. The remaining waste gas still containsammonia, however. Other examples are the waste gas of the atmosphericstorage tanks for aqueous ammonia or for urea solutions, the waste gasof screening equipment and centrifuges in the crystallization sectionsof urea plants and the like. A drawback of these streams is their lowpressure (for example atmospheric), since a low pressure isdisadvantageous in absorption processes.

[0023] It is increasingly required, for both economic and environmentalreasons, to clear these low-pressure ammonia-bearing streams of ammoniabefore they are vented to the atmosphere. Here, too, absorption in asuitable solvent, such as water or weakly ammoniacal water, is asuitable cleaning technique. Such absorption is commonly carried out inan absorber operating at low pressure and may be further optimized bydirect or indirect cooling by means of heat exchangers. ‘At lowpressure’ means at a pressure of between 0.1 MPa and 0.3 MPa but mostlyat virtually atmospheric pressure.

[0024] It has been found that the aforementioned drawbacks may beeliminated by increasing the pressure of the waste gases released in aurea plant at virtually atmospheric pressure with an ejector driven bythe waste gas from the high-pressure synthesis section and supplying thesaid waste gases to a medium-pressure absorber. The pressure of thewaste gases released at virtually atmospheric pressure is increased bybetween 0.15 MPa and 2 MPa, preferably between 0.2 and 0.5 MPa.

[0025] The expansion energy of the high-pressure waste gases is utilizedhere in an ejector by drawing in and compressing the low-pressure wastegases. As a consequence, the low-pressure waste gases arrive at a higherpressure level so that absorption of ammonia in the solvent is improved.The total ammonia losses from the urea plant are limited in this way.

[0026] In addition, the present process presents the advantage that thewaste gases supplied to the two absorption steps are combined, so thatthe absorption is combined in a single item of equipment, resulting inlower investment for the urea plant.

[0027] Furthermore, the present process is highly suitable for improvingand optimizing existing urea plants by increasing the pressure of thewaste gases from an absorber operating at virtually atmospheric pressurewith an ejector driven by the waste gas from the synthesis section andsupplying the said waste gases to a medium-pressure absorber. Thepressure of the waste gases from an absorber operating at virtuallyatmospheric pressure is increased here by between 0.15 MPa and 2 MPa,preferably between 0.2 and 0.5 MPa.

[0028] The invention may be applied in all existing urea processes, bothconventional urea processes and urea stripping processes, because wastegas streams of various pressure levels and unnecessary energy losses andpoor absorption results occur in all these processes.

[0029] Examples of conventional urea processes in which the inventionmay be applied are so-called ‘Once-Through’, Conventional ‘Recycling’and Heat Recycling Processes.

[0030] Examples of urea stripping processes in which the invention maybe applied are the CO₂ Strip process, the Ammonia Strip process, theSelfstripping process, the ACES (Advanced process for Cost and EnergySaving) process, the IDR (Isobaric-Double-Recycle) process and the HECprocess.

[0031] The invention is illustrated by the following example.

EXAMPLE

[0032] In a urea plant with a capacity of 1500 MT per day, the wastegases obtained at atmospheric pressure were compressed with an ejectordriven by the synthesis gas whereupon the compressed gases were absorbedin the existing 0.4 MPa absorber. As a result, the atmospheric absorberbecame redundant. Originally, there were two emission points in thisplant: one at the 0.4 MPa absorber and the other at the atmosphericabsorber. On account of the present process, one emission point waseliminated.

[0033] The process was carried out under the following conditions:

[0034] Synthesis gas as propellant for the ejector:

[0035] Flow rate: 1429 kilograms per hour

[0036] Pressure: 14 MPa

[0037] Temperature: 112° C.

[0038] Gas to be compressed (previously supplied to the atmosphericabsorber):

[0039] Flow rate: 807 kilograms per hour

[0040] Pressure: atmospheric

[0041] Temperature: 114° C.

[0042] Gas compressed by ejector at inlet of 0.4 MPa absorber:

[0043] Flow rate: 2636 kilograms per hour

[0044] Pressure: 0.4 MPa

[0045] Temperature: 106° C.

[0046] It was found that application of the present process resulted inammonia losses from this particular emission point of 2.45 kg/hour.

[0047] The ammonia losses for a 1500 MT per day plant with two emissionpoints amounted to 4.2 kg/hour.

1. Process for the preparation of urea from ammonia and carbon dioxide,characterized in that in a plant for the preparation of urea the wastegases released at virtually atmospheric pressure are compressed by anejector driven by the waste gas from the high-pressure synthesis and aresupplied to a medium-pressure absorber.
 2. Process according to claim 1,characterized in that the pressure of the waste gases released atvirtually atmospheric pressure is increased by between 0.15 and 2 MPa.3. Process according to claim 2, characterized in that the pressure ofthe waste gases released at virtually atmospheric pressure is increasedby between 0.2 and 0.5 MPa.
 4. Process for improving and optimizingexisting urea plants by increasing the pressure of the waste gases froman absorber operating at virtually atmospheric pressure with an ejectordriven by the waste gas from the synthesis and supplying the said wastegases to a medium-pressure absorber.
 5. Process according to claim 4,characterized in that the pressure of the waste gases from an absorberoperating at virtually atmospheric pressure is increased by between 0.15MPa and 2 MPa.
 6. Process according to claim 4, characterized in thatthe pressure of the waste gases from an absorber operating at virtuallyatmospheric pressure is increased by between 0.2 MPa and 0.5 MPa.